In the field of anesthesia, it has been known for a long time that anesthetics, which may be, for example, propofol, can be administered to the patient intravenously by means of a computer-assisted syringe pump. It is readily possible in this connection to control the syringe pump accurately, so that the quantity, i.e., the rate, of the anesthetic administered to the patient per unit of time can be adjusted accurately. During anesthesia the anesthetic in question is, however, continuously redistributed and broken down in the body of the patient, so that a blood concentration of the anesthetic becomes established, which cannot be easily determined from the quantity administered to the patient. It is known in this connection that this redistribution and breakdown process also has a very different course from patient to patient, even though the actually relevant patient data, such as height, body weight and age are almost in agreement.
Thus, statements can only be made with a very high inaccuracy with respect to the actually present blood concentration of the anesthetic in a specific patient. In addition, the problem arises that up to now it is not technically possible to monitor the propofol concentration in the blood continuously, for example, by means of sensors.
On the other hand, the concentration of the anesthetic in the blood is, however, correlated with the anesthesia and thus with whether or not the desired depth of anesthesia is actually reached. Since this parameter is not directly accessible, it is known from the state of the art to monitor the depth of anesthesia, for example, by means of EEG measurements during an intravenous administration of anesthetic. Depending on the depth of anesthesia determined in this case, the administration of the anesthetic and thus the syringe pump can be controlled.
In addition, it is known, for example, from DE 10 2012 203 897 A1 to describe the redistribution process of the anesthetic in the body of the patient within the framework of a so-called “three-compartment model,” wherein it is assumed that an exchange of the anesthetic, which is being administered intravenously, takes place, on the one hand, with the fatty tissue and connective tissue as well as, on the other hand, with the muscles, wherein the course of this exchange has a different rate in each case. In addition, a breakdown takes place at another rate. However, the portion of the anesthetic remaining in the blood circulation, which is not accessible based on measurement, as already explained, is relevant for the anesthesia effect.
It is additionally known from this publication that the rates, with which the anesthetic passes from the blood circulation into both compartments, i.e., the fatty tissue and connection tissue, on the one hand, and muscles, on the other hand, varies widely from patient to patient, which in turn leads to such a model and predictions based thereon to be marked by very great relative errors on the order of magnitude of 30%.
Since, however, the depth of anesthesia depends on the concentration of the anesthetic at the site of action, a control, which is supported alone on such a model, may lead to unnecessary problems. If an excessively high quantity of anesthetic is administered, this may lead to injuries in the patient. On the other hand, an excessively low quantity of anesthetic in the blood circulation may have the result that the patient is not sufficiently deeply anesthetized and possibly perceives parts of the surgical procedure carried out on him, which may in turn result in trauma.